a) Field of the Invention
The present invention relates to an objective lens system which uses a radial type gradient index lens element.
b) Description of the Prior Art
As conventional examples of lens systems having relatively wide field angles and high optical performance which are used, for example, as objective lens systems for endoscopes, there are known many lens systems such as a lens system disclosed by Japanese Patent Kokoku Publication No. Sho 60-46410 shown in FIG. 1. A conventional example shown in FIG. 1 is a lens system of the so-called retrofocus type which is composed, in order from the object side, of a front lens unit having negative refractive power, a stop and a rear lens unit having a positive refractive power. In this lens system, the front lens unit has a function to widen a field angle and another function to correct curvature of field by reducing a Petzval's sum of the lens system as a whole, the rear lens unit serves for suppressing production of spherical aberration and coma by distributing refractive powers among three positive lens elements and a cemented lens component, and the cemented lens component corrects lateral chromatic aberration which poses a problem, in particular, in a lens system which has a wide field angle. Though this conventional example favorably corrects aberrations, it is composed of lens elements in a number as large as six, thereby posing a problem of low producibility or a high manufacturing cost. Accordingly, it is expected to develop a retrofocus type objective lens system for endoscopes which is composed of an extremely small number of lens elements and has a high producibility. However, a lens system which favorably corrects aberrations and has high optical performance can hardly be composed of two homogenous spherical lens elements.
Further, an endoscope is generally equipped with a system for illuminating a location to be observed since it is used frequently for observing and photographing dark locations such as interiors of human bodies, aircraft engines, pipings and so on. In addition, an objective lens system for endoscopes has an NA which is not so large for obtaining a large depth of field. Accordingly, axial aberrations do not pose a serious problem, but a wide field angle and a negative-positive asymmetrical composition as shown in FIG. 1 make it difficult to correct offaxial aberrations. When an objective lens system is to be composed of two negative and positive lens elements, it is impossible to use a cemented lens component as in the objective lens system disclosed by Japanese Patent Kokoku Publication No. Sho 60-46410 mentioned above as the conventional example, whereby lateral chromatic aberration can hardly be corrected and offaxial imaging performance is remarkably degraded. It is therefore difficult to favorably correct offaxial aberrations with two homogenous lens elements so as to obtain favorable offaxial imaging performance.
It is therefore conceivable to use a radial type gradient index lens element which is characterized in that it corrects chromatic aberration in particular more favorably than a homogenous lens element. As a conventional example of an objective lens system for endoscopes which uses a radial type gradient index lens element and is composed of two lens elements, there is known a lens system disclosed by Japanese Patent Kokai Publication No. Sho 52-29238. However, this conventional example has a field angle as narrow as 72° which is insufficient for use as an objective lens system for endoscopes.
Further, a lens system disclosed by Japanese Patent Kokai Publication No. Hei 5-107471, for example, is known as a conventional example of an objective lens system having a wide field angle obtained with two lens elements. Though this example uses a radial type gradient index lens element, it does not effectively make use of the chromatic aberration correcting capability of the radial type gradient index lens element and does not sufficiently correct lateral chromatic aberration which poses a problem in a lens system having a wide field angle in particular.